The invention relates to a feeding device for air-flow-through crushing machines, also known as gas swept size reduction machines, which are used in particular to process materials consisting of cellulose such as precomminuted wood in the form of chips. Since such input material frequently originates from waste products, it can contain considerable amounts of foreign particles that may consist of metals, and also stones and sand. If such foreign particles enter the size reduction machines, they can cause considerable damage. This is especially the case with flaking machines, where the edges of the flaking knives are subject to rapid wear, if not damage. Furthermore, dull knives not only lessen the quality of the flakes produced but also cause a marked increase in power consumption during the flaking process which then results in frequent changes of the knives and all the costly inherent interruptions of production.
It has been known for a long time that, in order to reduce the damage and disadvantages caused by foreign particles, magnetically acting separation devices can be installed ahead of the intake of the size reduction machines. However, even if the magnetically acting separating devices function as intended they can remove only ferromagnetic foreign particles from the input material, while foreign particles made up of other substances such as stones, sand and soil remain unaffected.
In order to also remove non-ferrous foreign substances from the input material, it has been suggested in German application DE-OS 26 36 989, published February 1978, to install immediately ahead of the size reduction machine a separate feeding device in which an air current is used to separate the foreign particles. The air current produced by the paddle rotor of the size reduction machine, meets the input material as it is dropping inside a drop slide and carries the swirled material, largely by pneumatic action, into a central area of the paddle rotor. As part of this process, the foreign particles which are much more dense relative to the input process material should, as a result of their greater relative weight, pass across the sifting air current and in this way be eliminated from the process material. Whereas conventional feeding devices, functioning as pneumatic separators, have resulted in a certain reduction of damage and wear to the size reduction machines, it has become evident that larger particles of the process material are also disadvantageously eliminated contrary to intent, and smaller foreign particles still find their way into the size reduction machines.